Communications require two terminals, connected by a medium. In terrestrial communications, a telephone handset is generally connected to a telephone exchange by wires, fibre optics or coaxial cable. In terrestrial radio communications, a user terminal (otherwise, a handset) is in two way radio communications with a base station. In satellite communications, a user terminal (otherwise a handset) is coupled, by radio, to an radio base station on the earth's surface, through an intervening communications satellite.
In any of these systems, but especially in the two sorts of radio systems, a great deal of mutual transaction takes place between the user terminal and the base station before any meaningful call activity can take place. Firstly, the base station must register the user terminal. This requires an exchange of plural transactional messages and responses between the base station and the user terminal so that the base station can identify the user terminal, the user terminal can identify the base station, and so on. To commence a call, a further plurality of transactional messages and transactional responses must be exchanged between the base station and the user terminal whereby the user terminal becomes identified to the base station, the base station allocates a channel to the user terminal, the user terminal identifies the number to be called, the call is connected, and so on. All this happens before any voice or data communications can take place. Then, in the course of a call, the user terminal may require to switch base stations because the user terminal has moved out of the range of one and into the range of another. Yet another set of exchanges transactional messages and responses are required. Thereafter, the call must be terminated. A yet further set of transactional messages and responses is required to pass between the base station and the user terminal. Even when no activity is taking place, the "dormant" user terminal is required, periodically, or on changing area, automatically to reregister with the system. Here, again, a plethora of transactional messages and responses must pass between the user terminal and the base station, albeit without, in this instance, the knowledge or awareness of the possessor of the user terminal.
The problem becomes more acute when the access to and from the base station includes an intervening communications satellite. In terrestrial radio systems, such as GSM, time slots are allocated under a system known as TDMA (Time Division Multiple Access). Because the distance between a user terminal and a base station is generally small, the propagation delay is limited to just a few microseconds, and, for all practical purposes, the propagation delay can be counted as practically zero. However, in satellite systems, the round trip propagation delay, from earth to satellite and back to earth, can be quite large. In the case of a mid earth orbit system, such as is proposed for the ICO system, where the satellites are set into circular orbits at around 10,000 Km above the surface of the earth, the round trip delay can exceed one third of a second. In the case of geostationary communications satellites, which are set in geosynchronous equatorial orbits above the equator at a height of 25000 Km above the surface of the earth such that the nadir of each satellite stays stationary on the surface of the earth as the earth rotates, the round trip delay can approach one second. This creates severe timing problems with the TDMA timeslot allocation and introduces a further delay in call set-up. Satellite systems, unlike terrestrial systems, also attempt to locate the position of the user terminal on the surface of the earth before each call and on registration. This, too, involves a lot of mutual transactions between the user terminal and the satellite.
The routine is that one terminal (say, the handset or user terminal) sends a transactional message to the other terminal (say, the earth station via the satellite). A propagation delay ensues. The other terminal receives the transactional message. The first thing it does is to create an acknowledgement to the transactional message. This is stored in a buffer. When the next TDMA timeslot comes up, the earth station sends back the acknowledgement to the user terminal. A further propagation delay occurs. Meanwhile, the earth station has formulated the transactional response and placed it into a buffer. The next occasion when a TDMA transmission timeslot is available, which could be some little while away, the transactional response is sent back to the user terminal. And so it goes on, the user terminal and the earth station swapping transactional messages, acknowledgements and transactional responses, very like a game of Ping-Pong, until the required purpose has been achieved.
All of this imposes a large time and power overhead on the system. Even if the time taken is just a few seconds, the events occur, in the system, somewhere, nearly all of the time. The number of lost call hours per year to the system can be truly enormous. Many calls could be lost in a limited resource system such as satellite communications. The battery drain on the user terminal, most acute during transmission, can be unacceptably high. The same is true of a communications satellites, which have finite instant power available and may become over taxed at times of heavy traffic.
U.S. Pat. No. 4,910,733 discloses a local area network protocol in which an acknowledgement is dispersed with and the transactional response is itself used as an acknowledgement. However, the protocol requires the entire network to adopt the protocol and cannot operate both according to a conventional scheme with a separate acknowledgement signal, and the system which uses the transactional response as the acknowledgement. Thus the network is inflexible and not readily capable of expansion in capacity using both acknowledgement schemes.
The present invention seeks to simplify the transactions between the terminals (e.g., earth or base station and the user terminal, and indeed, in any other mutually signalling communications system) in a manner which is compatible with existing transactional protocols, without, in any way, reducing the total message or response value, and seeks thereby to save system time and to minimise battery and power drain.